The present invention relates generally to power supplies, and more particularly, to power supplies whose performance is responsive to the operating temperature of the power supplies.
A power supply is basically a voltage source that provides an input voltage to a particular circuit, device or component (hereinafter referred to collectively as "load"). In instances where the required input voltage of the load does not vary with changes in the operating temperature of the load, the power supply may be designed to provide a constant, temperature-independent output voltage. However, in situations where the required input voltage of a particular load varies with changes in operating temperature it is desirable that the performance of the power supply be temperature-dependant so that the output voltage of the power supply varies with the operating temperature of the power supply. Furthermore, in order to ensure the proper operation of the load over a range of temperatures, it may be highly desirable to have the output of the power supply match the input requirements of the load over a particular temperature range. To accomplish this, the output of the power supply and the input requirements of the load must vary by the same factor, or "temperature coefficient". It is the latter situation, namely, a power supply whose temperature coefficient is matched with the load's temperature coefficient, to which the present invention is directed.
A conventional power supply may have either a positive or negative temperature coefficient. The output voltage of a power supply with a positive temperature coefficient will increase as the operating temperature of the power supply increases and decrease as the operating temperature decreases. Conversely, the output voltage of a power supply with a negative temperature coefficient will decrease as the operating temperature of the power supply increases and increase as the operating temperature decreases.
The prior art contains several examples of power supplies that are designed to have temperature coefficients matched to the loads they supply. An example of one such power supply has one or more diodes stacked on a precise and substantially temperature independent voltage, such as a buffered bandgap voltage source. Together the stacked diodes and bandgap voltage provide the nominal output voltage of the power supply, while the diodes provide the power supply with a negative temperature coefficient. Unfortunately, this design does not offer much flexibility in designing the actual temperature coefficient or output of the power supply. Rather, the power supply's temperature coefficient is limited to a multiple of the diode temperature coefficients and the nominal output voltage of the power supply is limited to a combination of the bandgap voltage and the voltage across the stacked diodes. A second type of power supply found in the prior art includes a shunt regulator and a temperature compensation circuit. The shunt regulator provides the nominal output voltage of the power supply while the temperature compensation circuit provides the desired temperature coefficient. While this type of power supply provides design flexibility, the temperature compensation circuit is fairly complex and requires several components. A third type of power supply found in the prior art includes a positive temperature coefficient voltage source with feedback. Unfortunately, positive temperature coefficient sources are complicated and difficult to design. In addition, this type of power supply includes an additional resistor in the feedback path, which increases the number of components and, thereby, increases the manufacturing costs of the power supply.
Accordingly, there is a need for a power supply that requires few components and offers considerable design flexibility in selecting particular output voltages and temperature coefficients. The present invention is a power supply designed to achieve these results.